(1) Field of the Invention
The present invention relates to the drilling of geological formations and particularly to the perforation of the face of a coal seam. More specifically, this invention is directed to apparatus for forming boreholes in coal or the like and especially to hydraulic drilling apparatus. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in the mining of coal and especially in the formation of boreholes in the exposed face of a coal seam. The conventional method of forming such boreholes, which may be employed for the placement of explosives, has utilized mechanical drilling employing a spiral boring bar. Such boring bars may have a rod length of 1.5 m and be rotated either manually or by a carriage-mounted boring machine. If a boring machine is employed, the machine will be advanced towards the coal face either by hand or by a pneumatic drive.
Hand boring is a very laborious task, particularly in hard coal or other coals which may be bored only with difficulty. The use of carriage-mounted boring machines is often precluded by a lack of space and/or time.
In the interest of overcoming the problems briefly discussed above, it has been proposed to employ high pressure jets of water to form holes in a geological formation, particularly a coal face. Such proposed hydraulic drilling apparatus would employ a stationary nozzle to which the high pressure water was delivered via a conduit. The use of such water jets emanating from stationary nozzles has not proven to be a successful approach. It has additionally been proposed to modify the previously employed carriage-mounted boring machines so that the drill bit was replaced by a combined flushing and boring bit comprising nozzels to which high pressure water was delivered. Such a modified boring machine would, however, require a special rotatable boring bar which could tolerate water pressures of up to 350 bars and which would thus be expensive to produce. In addition, specially designed high pressure flushing heads would be required in order to insure that the water would penetrate into the bar which was rotated during the boring operation. The replacement of a mechanical boring bit by a nozzle head, which was itself mechanically rotated, will not result in any advantage or operational progress when compared to simple mechanical drilling.